In a digital radiography system, an x-ray image is captured by a digital sensor plate. Usually, the captured image is stored in a digital file in a computer system. When the image is stored in a digital file, information corresponding to the x-ray image can be stored in a header of the file. Information corresponding to the x-ray image could include, for example, the date the image was created, the patient's name, the patient's age, the physician's name, and hardware specifications of the digital radiography system.
The Digital Imaging and Communications in Medicine (DICOM) standards committee has published technical standards related to digital radiography (DICOM Supplement 32: Digital X-ray Supplement). This DICOM Supplement defines a standard for what information is to be included in the header of a file containing a digital x-ray image. According to this standard, the file header may include technical parameters of an anti-scatter grid used in a digital radiography system while capturing an x-ray image.
An anti-scatter grid is a physical device that blocks scattered radiation. When a primary x-ray beam interacts with a body, secondary x-rays are scattered in all directions. Secondary x-rays that are traveling in a direction other than that of the primary beam cause a radiographic fog in the x-ray image. Such radiographic fog reduces the contrast of the image.
An anti-scatter grid is comprised of alternating sections of radiopaque material (typically lead) and radiolucent material (typically aluminum), encased in a protective, radiolucent housing. An anti-scatter grid is designed to absorb only the x-rays traveling in a direction other than that of the primary beam.
Various technical parameters of an anti-scatter grid determine its effectiveness under different conditions. An anti-scatter grid may be parallel or focused. In a parallel grid, all of the radiopaque sections are parallel to each other, and perpendicular to the surface of the grid. In a focused grid, the radiopaque sections are progressively tilted such that straight lines extended from the points at which the sections intersect with the surface of the grid would intersect at a single point. This point is defined as the focal point of the grid. Parallel grids are less expensive to manufacture than focused grids, but have the undesirable effect of absorbing more of the primary x-rays. Focused grids absorb less primary radiation, but unlike parallel grids must be used at an appropriate focal distance from the beam source, plus or minus an acceptable margin of error.
Both parallel and focused grids may be linear or crossed. A linear grid is comprised of a single parallel or focused anti-scatter grid. A crossed grid is comprised of two linear grids, one on top of the other, such that the radiopaque sections of one grid are perpendicular to those of the other. Crossed grids absorb a significantly higher percentage of the scattered radiation than linear grids, but must be positioned much more carefully relative to the source of the x-ray beam. All grids may also be fixed in position, or moving. Moving grids are attached to a mechanism that is moved as the x-rays pass through the body being radiographed. This has the effect of minimizing, in the x-ray image, lines caused by the absorbence of primary x-rays by the grid.
Other technical parameters of the grid are the specific radiopaque and radiolucent materials used, the width of the sections of radiopaque material, the width of the sections of radiolucent material, the height of the grid, the ratio of the height of the grid to the width of the sections of radiopaque material (called the grid aspect ratio), the focal distance of the grid (relevant for focused grids only), and the period of time for which the grid is in motion while the digital sensor plate is being exposed to radiation (relevant for moving grids only). All of these factors determine the extent to which a grid will absorb secondary radiation, the extent to which a grid will undesirably absorb primary radiation, the proper range of focal distances for the grid, the tolerance of the grid for use outside of that range, and the dose of radiation to which the body being radiographed must be exposed in order to generate a useful x-ray image.
The DICOM standard specifies that the header of the file containing a digital x-ray image may contain various technical information concerning the grid that was used in the generation of the image. Such information comprises the type of the grid (e.g. focused, parallel, crossed, linear), the radiopaque material (e.g. lead, uranium), the radiolucent material (e.g. aluminum, plastic), the width of the radiopaque material in millimeters, the pitch of the radiopaque material in millimeters, the grid aspect ratio, the grid focal distance, and the period in milliseconds for which the grid moves during image generation (relevant for moving grids only).
It is useful to have a record of this information for multiple reasons. First, a record of the grid used in a specific x-ray procedure aids future medical research by providing data on the conditions of a past medical examination, the results of which are known. Second, it provides documentation that the correct grid for a specific procedure was utilized. Because different medical procedures require different focal distances and doses of radiation, different anti-scatter grids are best suited for different procedures. It is beneficial to health care providers to maintain a record that the correct grids were used for various procedures.
Presently, most radiography is executed using film. Thus, no digital file is created in which to store information concerning the examination. Instead, the radiography technician generally writes information concerning the examination on the film jacket in which the film is stored. Such information may or may not include the anti-scatter grid type. Because the process is entirely manual, it is highly subject to error and omission.
In an existing digital radiography system, a technician may manually enter information concerning the examination into a computer system for storage in the header of a digital file. Again, such information may or may not include the grid type, and is highly subject to error and omission. The technician may neglect to enter relevant information, the technician may make a typographical error, and the technician may be unaware of or mistaken concerning the detailed technical parameters of the anti-scatter grid that was utilized in the procedure. Manual entry of such information is also time-consuming.
Furthermore, it is possible for the technician to use the wrong anti-scatter grid during a procedure and not become aware of this mistake until viewing the resulting x-ray image. Such a mistake would be preventable if the digital radiography system were able to detect the use of an incorrect anti-scatter grid for the procedure being performed, and to alert the technician of the error.
What is needed is a way for a digital radiography system to identify an anti-scatter grid and its technical parameters, and to automatically store that information in the header of the file containing the digital x-ray image. Additionally, a way is needed for a digital radiography system to automatically detect when an anti-scatter grid being used is not the correct grid for the procedure being performed, and to alert the technician of the error.